1. Field of the Invention
The present invention relates generally to the field of molecular biology and protein engineering. More particularly, it concerns compositions and methods for producing and screening polypeptides including non-canonical amino acids.
2. Description of Related Art
Life on Earth has been shaped by the universal and almost static genetic code set early in evolutionary history. At the coding level, life's proteome has been confined to20 canonical amino acids, as well as selenocysteine and pyrrolysine, two rare, naturally occurring non-canonical amino acids (NCAAs) with specialized incorporation pathways. Because of differences in the way in which tRNA molecules are charged by tRNA synthetases in different organisms, it is difficult to predict to what extent a tRNA charged with a non-canonical amino acid by an engineered tRNA synthetase will truly be orthogonal to the canonical set of tRNAs and tRNA synthetases in any given organism. There is frequently background charging of introduced tRNAs by native tRNA synthetases, and background charging of native tRNAs by introduced tRNA synthetases. These effects likely contribute to the rapid loss of any introduced system to allow for incorporation of a NCAA. Thus, to date, there remains a need for a stable system that provides incorporation of NCAAs and that can be applied to a broad range of host organisms.